1. Field of the Invention.
This invention relates to a carbonaceous fuel produced from fluid petroleum coke and a method for its preparation.
2. Description of the Prior Art.
Fluid coke is the product of a known coking process for pyrolytically upgrading petroleum oils which utilizes the fluidized solids technique. In this process, a fluidized mass of high temperature solids, usually coke formed by the process, supplies the heat of pyrolysis. The charging stock to be upgraded is sprayed into the fluid bed and upon contact with the high temperature solids undergoes pyrolysis, evolving lighter hydrocarbon vapors and depositing carbonaceous residue, i.e., coke, on the solids. To maintain the coking temperature, a portion of the solids is continuously circulated to an external heating zone and returned to the fluidized bed of solids present in the coking zone.
The coke particles withdrawn from the coking zone are reheated by partial combustion in the heating zone. Thus the fluid coke product is the result of sequential oxidations of small increments of exterior particle surface with intermediate depositions of fresh coke. In a typical process, about 4% wt. to 7% wt., based on charge, of the gross coke produced in the coker is removed in the burning zone and about 1 to 5% by weight of the coke returned to the coker is added in each subsequent passage through the coking reaction zone.
A fraction of the circulating coke solids is withdrawn, usually on a continuous basis, as a fluid coke product, the withdrawal maintaining a substantially constant inventory of solids in the process and maintaining the desired coke particle size range. The coke produced in fluid coking systems is very hard. Because of its hardness, density, sulfur content, etc., it is not generally of a quality suitable for use in most carbon utilizing applications. It has been further asserted that the burning properties of fluid coke are inferior to those of other solid carbonaceous materials such as bituminous coal and delayed petroleum coke. Certainly, the tendency of fluid coke to have a higher sulfur content than these other carbonaceous fuels creates pollution problems during its combustion. But the less desirable burning properties of fluid coke also include its relative combustibility, i.e., the ease with which it will burn.
The following table shows a comparison of fluid coke with other solid carbonaceous fuels.
TABLE I ______________________________________ Low- Delayed Volatile Sub- Petroleum Petrol- Bitum- bitum- Fluid eum inous inous Coke Coke Coal (1) Coal (2) ______________________________________ Volatile (wt. %) 3.7-7.0 8-18 18.2 33.3 Ash (wt. %) 0.05-1.6 0.1-2.8 3.9 3.8 Heating Value (BTU/lb.) 13,900-14,400 14,550 9,420 H (wt. %) 1.6-2.1 4.8 6.4 C (wt. %) 88-95 84.0 54.6 S (wt. %) 1.5-10 0.6 0.4 Ash-softening Temp. (.degree.F.) 220-2800 2060 Grindability 20-30 40-60 ______________________________________ (1) West Virginia, Pocahontas No. 3 (2) Wyoming, Monarch
The ignition temperature of fluid coke is generally about 1400.degree. F., considerably higher than coal or delayed petroleum coke (about 1150.degree. F.). The burning rate of fluid coke, however, can be equal to or greater than that of delayed petroleum coke, indicating that, once it is ignited and ignition is maintained, fluid coke combustion may be completed in a comparable or shorter amount of time than delayed coke combustion.
Fluid coke does have potential for use as a solid fuel. However, its use in this regard is limited by the foregoing properties, especially its high ignition temperature and the difficulty of maintaining ignition in low heat capacity furnace systems such as steam boilers.
Steam generating boilers burning pulverized solid fuel are well known in the art. Pulverized fuel and air are blown into a furnace combustion chamber as a combustible mixture and ignited. The resulting solid fuel-air flames extend a material distance into the chamber, the walls of which are kept comparatively cool by heat transfer to water tubes lining the chamber (unlike relatively large heat capacity furnace systems such as kilns which retain heat and are less sensitive to temperature variations). Thus, the steam boiler combustion chamber does not "run hot" for ensuing reignition or stable combustion in the event of process fluctuations such as momentary flame die-out or instability.
Accordingly, transient conditions such as clogging of the pulverized solid fuel feeder means, pulverizer discontinuity, decreased air supply, etc., that reduce or briefly curtail the fuel or air supply tend to cause flame instability and possible flame-out, which in turn can result in furnace shutdown with loss of steam generating capacity and production time. Restarting a furnace after flame-out and shut-down is complicated by the need to clean the combustion chamber of all gases, combustion products and unburned mixtures before reignition. This not only consumes valuable production time but also results in objectionable stack emissions that tend to foul pollution control equipment.
When fluid coke is employed as the solid fuel, the foregoing problems are aggravated because of the difficulty of maintaining ignition and the consequently greater flame instability. A related problem regarding fluid coke combustion in relatively small heat capacity furnace systems, is unburned carbon loss from the furnace system.
A known technique for maintaining combustion temperatures in pulverized solid fuel burners is the use of an auxilliary fuel as an ignition accelerator. The auxilliary fuel which may be gaseous, liquid or solid is premixed with either the primary air or the pulverized solid fuel as it enters the burner nozzle. Another method is to position a pilot flame adjacent to the nozzle of the pulverized fuel burner. Auxilliary fuel is used to maintain the pilot flame which aids in maintaining the temperature of the solids sufficiently high to promote complete combustion in the combustion zone.
Again, because of the high ignition temperature of fluid coke and the temperature sensitivity of its combustion, neither of these methods entirely satisfies the problems encountered when burning fluid coke in relatively small heat capacity furnace systems. The problem is not only the maintenance of ignition temperatures at the burner itself but throughout the combustion chamber.
Accordingly, an object of the present invention is an improved fluid petroleum coke fuel. A related object is a fluid petroleum coke fuel having properties such that it is a superior fuel in relatively small heat capacity furnace systems such as steam generating boilers, especially industrial boilers. Other objects will be apparent from the following description of the present invention.